# Measuring voltage and current in a DC circuit

This material is replicated on a number of sites as part of the SERC Pedagogic Service Project
Initial Publication Date: July 30, 2007

## Summary

Students frequently confuse how to measure voltage and current in an electrical circuit. In part, students may not understand how a multi-meter measures voltage and current. With such conceptual issues at hand, many students record circuit data that has been measured incorrectly especially in an initial circuits laboratory setting. Incorrect data then prevent these students from seeing the patterns that exist among parallel and series circuit measurements of current and voltage.

This demonstration allows the instructor to model measuring voltage and current in a lab but using a virtual circuit simulator that can be displayed on a screen in any size classroom via an LCD projector and computer. Therefore, such a demonstration can be executed from within an instructor's Power Point presentation in the classroom and can include student interaction with the instructor as circuit measurement is explored. The activities are designed to enable students to correctly measure voltage and current in a simple-circuit laboratory or using a circuit simulator such as PhET.

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## Learning Goals

The goal of this activity is to develop understanding of current and voltage in simple DC circuits related to proper measurement. Specifically, students will demonstrate the following: - proper measurement of DC current in simulations, circuit diagrams, and/or laboratory settings. - proper measurement of DC voltage in simulations, circuit diagrams, and/or laboratory settings. - understanding of the flow of current (and its relationship to measuring current in series). - understanding of parallel circuits and its relationship to measuring voltage in parallel. Where simple AC circuits are included in the curriculum, students will also demonstrate: - proper measurement of AC current in simulations, circuit diagrams, and/or laboratory settings. - proper measurement of AC voltage in simulations, circuit diagrams, and/or laboratory settings.

## Context for Use

These questions are to be used for in-class peer instruction. Students should make predictions, share predictions, and discuss and make a new prediction. There should also be a general class discussion/presentation after each question to clarify the material. These activities are appropriate in a variety of settings as described below:

Educational level: Conceptual physics to an upper level DC circuits class
Setting: These activities can be used as a pre-lab activity or in the classroom; The activities are appropriate for small classes to large lecture hall classes.
Time required: Instructors may steam-line or expand these activities. A minimum time of 15 minutes should be allotted for these activities.
Special equipment: computer & LCD projector
Pre-requisite knowledge: Students should have had an introduction to current, voltage, and resistance either from reading, previous class work, or discussions.

## Description and Teaching Materials

The demonstration aspects of these questions should be done with a virtual experimental set up in order the potential of damaging lab equipment. The virtual lab has the additional advantage that students can perform similar experiments if or when they can.

The virtual experiment can be done with the PhET Circuit Simulator from the University of Colorado. The simulation PhET software can be downloaded for free from http://phet.colorado.edu/web-pages/index.html and run without internet access. Because the software can be accessed for free, instructors can have students work on these activities outside of the classroom or laboratory.

The questions, descriptions of the misconceptions often found with these questions, and a short description of how to run the labs, are contained in the two files:

Instructor guide for current measurement questions & demos. (Microsoft Word 445kB Jul29 07)
Instructor guide for voltage measurement questions & demos. (Microsoft Word 183kB Jul29 07)

## Teaching Notes and Tips

See discussions included with each question.

Students must become engaged in the class/lab activity in order for us to help them learn. Students should commit to an answer before the demonstration, either by written responses or some manner of class-room voting (classroom response, raised hands, etc). Peer Instruction, where students can learn from each other before re-voting, has proven to be an effective tool for engaging students.

The PhET software allows the instructor to return to the simulation to demonstrate ideas that students are struggling with. If access to computers is available, student investigation using PhET can be substituted for instructor demonstration.

## Assessment

Similar questions can be asked within homework, quiz, or test assessment items. In these situations, students may be asked to draw a simple circuit that includes a voltmeter and ammeter connected properly to measure voltage and current, respectively, for a specific resistor or other component.

Students may also be asked to describe what happens when one of these meters is connected in a specific way (e.g. ammeter connected in parallel with the resistor results in what type of situation?).

## References and Resources

Results of studies on the effectiveness of PhET simulations, and presentations given about the PhET materials are available at https://phet.colorado.edu/en/research.